The present invention relates to a miss convergence compensating device for a projection type display.
It is a general process in a projection type display to arrange in parallel three projection tubes for red, green and blue colors together with a projection lens and synthesize projected color images on one screen.
FIG. 1 illustrates a relationship between projection lenses and an image projected on the screen in a general projection type display.
In FIG. 1, reference numerals 200, 201, 202 designate projection lenses for three colors; 210 designates a screen and .omega. is gradient of blue light in relation to green light.
As shown in FIG. 1, on the occasion of projecting and synthesizing color images, the red and blue images are projected in a direction inclined to the screen and therefore a trapezoidal distortion in a vertical direction (trapezoidal distortion with a different length on two sides in the vertical direction) is generated based on the projective geometry and it causes miss convergence.
In view of compensating for such miss convergence, a convergence yoke (hereinafter abbreviated as CY) for auxiliary deflection of an electron beam similar to a deflection yoke (hereinafter abbreviated as DY) is mounted to the neck part of the projection tube and an output from the convergence amplifier is applied to CY for compensating for miss convergence.
The major factor of miss convergence resulting from such projection type optical system (projection lens, etc.) includes the vertical trapezoidal distortion (hereinafter referred to as Vertical Keystone, V-KS) and horizontal linearity distortion (hereinafter abbreviated as H-LIN).
FIG. 2 indicates a positional relationship between the projection tube and DY, CY in a general projection type display.
In FIG. 2, reference numeral 16 designates the projection tube; 17, DY; 18, CY; and 19, an electron gun.
Deflection sensitivity of DY 17, CY 18 can be expressed in terms of the deflection distance of the electron beam for a unit electromagnetic energy input and is proportional to the effective magnetic path of each yoke (longitudinal direction of the neck part).
Meanwhile, as shown in FIG. 2, the effective magnetic path length of CY 18 is about 1/2 of such length of DY 17 because there is a limitation on the size in order to prevent adverse effect by CY 18 on the electron lens part of the electron gun 19 which is provided adjacent to the left side of CY 18.
Therefore, the deflection sensitivity of CY 18 is as bad as about 1/2 in comparison with DY 17.
As described above, the deflection sensitivity of CY is as bad as about 1/2 that of DY and miss convergence compensation is carried out by CY in the prior art, resulting in a problem that a large amount of electrical power is consumed for compensation of miss convergence.
The applicant of the present invention has the following three U.S. patents and patent applications in total regarding miss convergence compensation of projection type display. The former two patents relate to improvement of the deflection yoke itself and the latter application relates to a measure for saturation of an output amplifier.
U.S. Pat. No. 4,420,734 PA1 U.S. Pat. No. 4,433,318 PA1 U.S. Ser. No. 084,125 (Aug. 12, 1987) now U.S. Pat. No. 4,814,671